1. Field of the Invention
This invention relates to the continuous hot rolling of ferrous long products, including, inter alia, rounds, octagons, squares and the like.
2. Description of the Prior Art
As herein employed in the rolling of rounds, the term xe2x80x9csizingxe2x80x9d means imparting a final deformation during the last stage of rolling to obtain a finished nominal product diameter within a specified standard tolerance which is typically about xc2x10.1 mm diameter tolerance and 0.1 mm ovality or better. Also, as herein employed, the term xe2x80x9cfree sizingxe2x80x9d means making adjustments to the roll partings of sizing stands to produce finished product diameters which are slightly larger or slightly smaller than the nominal diameter designated for the roll grooves, but are diameters which are within an acceptable tolerance for the obtained diameter.
Various techniques have been developed for sizing and free sizing ferrous long products. For example, as disclosed in U.S. Pat. No. 4,907,438 issued Mar. 13, 1990 to Sasaki et al., it is known to roll round process sections through successive two roll sizing stands, with a round-round pass sequence, and with the roll passes configured to take relatively light reductions on the order of 8-15% per pass.
By feeding the sizing stands with different diameter rounds taken from different stands in the upstream intermediate or finishing sections of the mill, and by changing roll diameters and groove configurations, a range of products can be sized.
Some free sizing is also possible, albeit within a relatively narrow range, due to the limitations imposed by the spread which inevitably accompanies rolling in two roll passes.
A further drawback with the Sasaki et al. round-round pass sequence is the development in certain products of a duplex microstructure, where the grains throughout the cross section of the product vary in size by more than about 2 ASTM grain size numbers (measured in accordance with ASTM E112-84).
It is generally recognized that a variation of more than about 2 ASTM grain size numbers in the cross section of a product can cause rupturing and surface tearing when the product is subjected to subsequent bending and cold drawing operations. Such grain size variations also contribute to poor annealed properties, which in turn adversely affect cold deformation processes.
The development of duplex microstructures was subsequently recognized as stemming from the inability of the light reduction round sizing passes to achieve adequate deformation throughout the product cross section within a sufficiently short time. This problem was addressed by the technique described in U.S. Pat. No. 5,325,697 issued July 5, 1994 to Shore et al. Here, a two roll round-round light reduction sizing sequence is immediately preceded by a heavy reduction two roll oval-round pass sequence. The heavy reductions taken in the oval-round pass sequence produce a deformation pattern penetrating to the center of the product with high strains. Before the accompanying stresses are relieved through microstructural recrystallization and recovery, rolling continues in the immediately succeeding light reduction two roll passes.
In effect, therefore, the reductions taken in the four successive passes comprise one substantially continuous process, with a resulting strain pattern across the product cross section which avoids the development of a duplex microstructure.
Here again, however, the available range of free sizing rolling is limited due to the spread experienced when rolling in two roll passes.
It is also known to employ three and four roll passes in round-round sizing sequences. These afford a wider range of free size rolling because the products are more closely confined in the roll passes and thus do not experience the degree of spread encountered in two roll passes.
However, as compared to two roll passes, three and four roll passes are far less efficient in achieving sufficient penetration of deformation to the center of the product. Such penetration is required to obtain a uniform grain structure from center to surface of the product. This is particularly important for products which develop their properties from grain refinement.
There exists a need, therefore, for an improved method of hot rolling long products, which is capable of achieving sizing tolerances and substantially uniform center to surface grain structures, and which also has a broadened range of free sizing. It is to these ends that the present invention is directed.
In accordance with a preferred embodiment of the present invention, a round ferrous process section is initially rolled in first and second two roll passes at an elevated temperature of between about 650 to 1000xc2x0 C. to effect a combined heavy reduction in cross sectional area of at least about 20-55%, with an accompanying effective strain pattern dominated by a concentration of maximum effective strain at a central region of the product""s cross section. Prior to the occurrence of microstructural changes due to recrystallization and recovery and while the effective strain pattern remains dominated by a concentration of maximum effective strain at a central region of the product""s cross section, the product is rolled in at least third and fourth roll passes, each being defined by at least three rolls, to effect a further combined relatively light reduction in product cross sectional area of not more than about 4-25%.
When rolling a round process section into a finished round product in the above manner, e.g., a rod or bar, the first roll pass produces an oval cross section and the second roll pass produces a round process cross section.
The third and fourth roll passes complete the shaping of the process round cross section into a finished round having no more than xc2x10.1 mm diameter tolerance and 0.1 mm ovality, or xc2xc ASTM Rod or Bar tolerance, whichever is better. After cooling to a state of thermal equilibrium, the resulting product will have a grain size variation across its cross section of not more than about 2 ASTM grain size numbers.